Magnetic field sensors based on semiconductor thin films, either employing the Hall effect or their (ordinary) magnetoresistance are sensitive to magnetic fields that are directed perpendicular to the plane of the film. For a range of applications, such as brushless permanent magnet electromotors, crank shaft and cam shaft position sensors and ABS (acceleration-decelaration) sensors in automotive systems, this is often the preferred orientation in view of spatial limitations (miniaturization). The sensistivity of the known devices is, however, limited, and there is an ever increasing need for higher sensitivity.
An object of the present invention is to realize semiconductor thin film magnetic field sensors for measuring a component of a magnetic field perpendicular to the plane of the film with useful, and therefore increased, sensitivity.
To this end the magnetic field sensor according to the invention comprises a giant magnetoresistance (GMR) material or a spin tunnel junction. In the sensor in accordance with the invention the perpendicular sensitivity, i.e. sensitivity for a component of the applied magnetic field perpendicular to the plane of the film, is based on the GMR effect and/or spin tunnel junctions. Sensors based on the giant magnetoresistance (GMR) or the spin tunneling effect (also called tunnel magnetoresistance (TMR)) over.a strongly improved sensitivity in particular to the component of magnetic field perpendicular to the films. However, all designs proposed so far show useful sensitivity for a magnetic field in the plane of the films only. The invention makes it possible to provide GMR and spin tunneling based sensors (with useful perpendicular-field, as well as in-plane, sensitivities), all based on the same physical principles, with the advantage of the high sensitivity offered by these technologies.
A spin tunnel junction (STJ) comprise a layered structure F1/I/F2, where F1 and F2 are layers which may be laminated, but which at least contain one ferromagnetic layer, and where I is an insulating barrier layer. The tunnel current is directed perpendicular to the plane of the layers. The GMR layered structures that we consider here comprise a similar layered structure F1/M/F2, where F1 and F2 are layers which may be laminated, but which contain at least one ferromagnetic layer, and where M is a non-ferromagnetic (often: non-magnetic) separation layer. In principle either the Current In the Plane of the layers (CIP) geometry or the Current Perpendicular to the Plane (CPP) geometry can be used. However, in practice the much larger resistance change combined with the more simple manufacturing process will favour the use of the CIP geometry in sensors.
Sensitivity to a perpendicularly directed magnetic field (more generally: to the perpendicular component of an applied magnetic field) is particularly high in sensors having F1 and F2 layers with a strongly different uniaxial anisotropy, and/or with a modified magnetization curve. A modified curve can be realized by antiferromagnetic coupling with an auxiliary ferromagnetic layer. More specifically it is advantageous to select the field range within which the element is sensitive by making use of uniaxial interface anisotropy, either easy-plane or easy-axis, and/or to realize step-shaped resistance field (R(H)) curves by making use of antiferromagnetic interlayer exchange coupling, and/or to strongly increase the sensitivity of the sensors by providing an auxiliary ferromagnetic layer. Antiferromagnetic interlayer exchange coupling increases the sensitivity.
The various ways used to optimize the magnetization curves of the layers that determine the electrical resistance are sometimes referred to in the literature as xe2x80x98spin engineeringxe2x80x99. The sensor designs according to the invention are preferably based on spin tunnel junctions, because for such systems only the magnetization direction of the magnetic layer closest to the oxide barrier layer determines the magnetoresistance (as far as is known at the present). The use of auxiliary magnetic layers situated away from that interface does not affect the electrical resistance. Nevertheless, with suitably chosen auxiliary layers (high resistivity) the same ideas can also be applied to GMR systems. Although the text below is focused on the application to spin tunnel junctions, the same solution applies for sensors based on GMR effects.